Most dealers usually have a box of different jets... maybe the kid was just clueless. If you are really stuck and need to find some - try DGY [800.915.7878] they should have any jets in stock that you should need.
If you are good down low - that is mostly your pilot jet and air screw. If you are good up top - that is mostly your main jet. The middle is a combo of the needle, clip position on the needle, and power jet. Without even changing jets, you can move the clip on the needle up to lean out your mid range or move the clip down on the needle to richen up the middle. I believe cutting out and missing means you're too lean (which is a little unusual if your bike is at stock jetting, but very possible). So maybe try moving the clip down one or two positions on the needle before you do anything else.
I think on the 01 YZ 250 the stock jettting is a 50 pilot, 50 power jet, 178 main, and N3EJ needle, with the clip at the second from top position.
Good luck :thumb: